In a follow-up to reports on the Legionnaires’ disease outbreak in Hopkins, Minnesota, the Minnesota Department of Health (MDH) said Wednesday that the likely source of the Legionnaires’ disease outbreak in Hopkins as a set of cooling towers at Citrus Systems, Inc., located at 415 11th Avenue South, Hopkins.

The outbreak sickened a total of 23 people; 17 were hospitalized and one person died.

Legionella/CDC

MDH investigators based their conclusion on the distribution of cases and their exposure histories (where they live, work, or otherwise were in Hopkins) and the test results of water samples taken from the Citrus Systems cooling towers. Laboratory testing showed the strain of Legionella bacteria found in one of the cooling towers matched the strain of bacteria in samples taken from patients linked to the outbreak. Legionella bacteria cause Legionnaires’ disease. Other cooling towers in the outbreak area were sampled, but did not have the specificLegionella strain identified.

MDH, working with Hennepin County Public Health and the City of Hopkins, confirmed on Sept. 26 that Citrus Systems had cooling towers, following a tip from the public. Water samples were collected and the towers were chemically cleaned and disinfected on Sept. 27. The MDH Public Health Laboratory found positive bacterial growth for Legionella from one of Citrus Systems’ two towers. Isolates were then analyzed further, using pulsed-field gel electrophoresis testing and whole genome sequencing to obtain a DNA fingerprint of the specific strain. The DNA fingerprint of the isolates exactly matched the DNA fingerprint of Legionella isolates from four patients. New York’s Wadsworth Public Health Laboratory and the Centers for Disease Control and Prevention in Atlanta (CDC) assisted in analyzing and comparing the samples.

“The lab testing is highly specific and provides us with great confidence that this tower is the source of infection,” said MDH Deputy State Epidemiologist Richard Danila. Overall, the geographic distribution of cases was consistent with community-wide exposure to a contaminated aerosol. Patient and family interviews did not identify a common exposure among the cases other than living, working or spending time in the outbreak area.

Health officials note that food products made and packaged at the Citrus facility in Hopkins are not affected by nor involved in the outbreak of Legionnaires’ disease in that community.

Legionella bacteria do not cause infection through eating food or drinking water or beverages. Infection is only caused through inhalation of fine water droplets containing the bacteria.

The last case became ill on Sept. 22. There have not been any further outbreak-related cases of Legionnaires’ disease.

“Although it’s been more than 14 days since the tower was sanitized and since our last case, which is beyond the typical 10-day incubation period forLegionella, it’s still possible we could see an additional case if someone were exposed on Sept. 27,” Danila said. “It often takes time for cases to present for care and be tested. However, we believe the source has been identified and remediation has occurred.”

Citrus Systems has cooperated fully with all agencies involved in the investigation, cleaning and disinfecting its towers promptly after being contacted by health officials. The company is working with MDH to ensure their system is working correctly. MDH has recommended retesting of the cooling tower so that any problems can be quickly identified.

Health officials noted that Citrus has a management plan in place for its cooling towers and works with a qualified contractor for the ongoing operation and maintenance of the system.

“We don’t know in this case exactly what went wrong to cause the build-up of Legionella,” Danila said. “This situation clearly demonstrates how difficult it can be to keep systems free of Legionella, even when they are well-maintained.”